GB2266785A

GB2266785A - Holographic laser scanner for a laser printer

Info

Publication number: GB2266785A
Application number: GB9308398A
Authority: GB
Inventors: Keun Young Yang; Sung-Woo Noh
Original assignee: Gold Star Co Ltd
Current assignee: LG Electronics Inc
Priority date: 1992-04-24
Filing date: 1993-04-23
Publication date: 1993-11-10
Anticipated expiration: 2013-04-23
Also published as: JPH06102465A; KR930021398A; GB2266785B; KR960010411B1; GB9308398D0

Abstract

A holographic laser scanning unit for a laser printer comprises a laser diode 1, a collimating hologram 2, a rotatable hologram disc 3, a hologram disc drive motor 4, a mirror 5, a concave mirror 6, and a wavelength compensating hologram 7 for compensating movement of a condensed position of the laser beam due to wavelength variation and condensing the laser beam to a drum 8. A laser scanning start point detecting optical system includes a laser scanning start point detecting hologram 9 between mirrors 5, 6, and an optical detector 11 with a slit 10 between the hologram 9 and the optical detector 11. <IMAGE>

Description

HOLOGRAPHIC LASER SCANNING UNIT FOR LASER PRINTER BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to a laser scanning system for a laser printer, and more particularly to a laser scanning unit employing a holographic theory for the laser pinter.

Description of the Prior Art With reference to drawings, Fig. 1 is a plan view of a laser scanning unit of a conventional laser printer and Fig.

2 is a front view of the laser scanning unit. The known laser scanning unit includes a laser source 111 which is turned on/off under the control of a controller 200 to generate a laser beam. Disposed in series on the optical path at the front of the laser source 111 are a collimator 112 and a cylindrical lens 113. The collimator 112 collimates the laser beam to convert it into a parallel beam, while the cylindrical lens 113 condenses the parallel beam of the collimator 112 to a rotating polygonal mirror 114. The rotating polygonal mirror 114 is rotated by a drive motor 115 and reflects the condensed laser beam of the cylindrical lens 113 to a scanning plane at a predetermined angle. The laser scanning unit further includes a compensating lens system, having two lenses 116 and 117, as well as a reflection mirror 118.The compensating lenses 115 and 117 compensate the vertical movement of the scanned laser beam in the scanning plane. The reflection mirror 118 reflects the compensated laser beam of the compensating lenses 116 and 117 and scans it to a drum 300.

In the above laser scanning unit, at least one toric lens having a toric surface should be used for the compensating lens system.

In operation of the above laser scanning unit, the laser source 111 is turned on under the control of the controller 200 to generate the leaser beam which is in turn collimated by the collimator 112. The collimated laser beam, that is, the parallel laser beam is, thereafter, condensed and scanned by the cylindrical lens 113 to a reflection surface of the rotating polygonal mirror 114. Since the polygonal mirror 114 is rotated by the drive motor 115, the incident laser beam is reflected by the reflection surface of the mirror 114 to the scanning plane having the predetermined angle and, thereafter, compensated by the compensating lenses 116 and 117. The compensated laser beam is in turn reflected by the reflection mirror 118 and scanned to the drum 300.

When the laser beam is reflected to the scanning plane by the rotating polygonal mirror 114, it typically slightly moves in a direction vertical to the scanning plane due to a mirror surface inclination, caused by machining error of the polygonal mirror 114, as well as rotational vibration generated in the rotating polygonal mirror 114. Such a vertical movement of the laser beam with respect to the scanning plane is compensated by the compensating lenses 116 and 117 constituting the compensating lens system. The compensating lenses 116 and 117 also converts the curved focus tracking of the scanned laser beam into the linear focus tracking.

As described above, in the known laser scanning unit, it is required to compensate the vertical movement of the laser beam with respect to the scanning plane and to convert the curved focus tracking of the laser beam into the linear focus tracking, and to maintain a predetermined scanning velocity of the laser beam on the drum. Hence, at least one toric lens having the toric surface should be used as the compensating lens of the compensating lens system. However, as well known to those skilled in the art, it is very difficult to machine the toric lens having the toric surface. Thus, the known laser scanning unit has a problem in that it involves capital investment caused by necessity of the toric lens.

SUMMARY OF THE INVENTION it is, therefore, an object of the present invention to provide a holographic laser scanning unit for a laser printer in which the aforementioned problem can be overcome and which ;as an optical system using a mass-producible hologram plate instead of the conventional toric lens and the rotary polygonal mirror, thereby reducing the manufacturing cost and improving the picture resolution.

to accomplish the above-mentioned and other objects, a holographic laser scanning unit of the present invention comprises a laser scanning optical system and a laser scanning point detecting optical system.

The laser scanning optical system includes a laser diode or generating a laser beam, a collimating hologram for collimating the laser beam of the laser diode, a rotatable hologram disc having at least one hologram plate and diffracting the laser beam of the collimating hologram at a predetermined angle, and condensing and scanning the diffracted laser beam, a hologram disc drive motor, a reflection mirror for reflecting the laser beam scanned by the hologram disO, a concave reflection mirror for recondensing and reflecting the laser beam reflected by the reflection mirror, and a wavelength compensating hologram for compensating movement of a condensed position of the laser beam, reflected by the concave reflection mirror, due to wavelength variation of the laser beam and condensing the laser beam to a drum.

The laser scanning start point detecting optical system includes a laser scanning start point detecting hologram for receiving the laser beam, diffracted, condensed and scanned by the hologram disc and reflected by the reflection mirror, at a position before the laser beam is received by the concave reflection mirror, a slit and an optical detector which are arranged in order.

In accordance with the present invention, the laser beam generated by the laser diode is collimated by the collimating hologram and received by the rotating hologram disc. The incident laser beam is diffracted by the rotating hologram disc at a predetermined diffracting angle and scanned to the concave reflection mirror by way of the reflection mirror. At the concave reflection mirror, the laser beam is condensed and reflected to be scanned to the drum by way of the wavelength compensating hologram. In this case, the laser beam is received by the laser scanning start point detecting hologram prior to its incidence to the concave reflection mirror. Upon reception of the laser beam, the scanning start point detecting hologram condenses the laser beam to the optical detector which detects the laser beam and outputs a laser scanning start signal to the microprocessor.

In response to the laser scanning start signal, the microprocessor turns on/off the laser diode in accordance with a video signal. The laser beam generated by the laser diode is scanned to the drum to form a latent image on the drum surface.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Figs. 1 and 2 show a laser scanning unit according to the prior art, in which: Fig. 1 is a plan view; and Fig. 2 is a front view; Fig. 3 is a front view of a holographic laser scanning optical system according to an embodiment of the present invention; Fig. 4 is a side view of a laser scanning unit, comprising the holographic laser scanning optical system of Fig. 3 and a laser scanning start point detecting optical system, in accordance with the present invention; Fig. 5 is an enlarged view of the circled part A of Fig.

3; Fig. 6 is an enlarged view of the circled part B of Fig.

3; Fig. 7 is a schematic view showing a manufacturing theory of a collimating hologram in accordance with the present invention; Fig. 8 is a schematic view showing a manufacturing theory of a hologram piece constituting a hologram disc in accordance with the present invention; Fig. 9 is a schematic view showing a manufacturing theory of a laser scanning start point detecting hologram in accordance with the present invention; and Fig. 10 is a schematic view showing a manufacturing theory of a wavelength compensating hologram in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to drawings, Fig. 3 shows a front view of a holographic laser scanning optical system according to the present invention and Fig. 4 is a side view of a laser scanning unit, manufactured by combination of the laser scanning optical system with a laser scanning start point detecting optical system, in accordance with the present invention. The holographic laser scanning unit comprises two optical systems, that is, the laser scanning optical system and the scanning start point detecting optical system.

As shown in Fig. 3, the laser scanning optical system includes a laser diode 1 7 generating a laser beam under the control of a microprocessor 12 in response to a printing video signal, and a collimating hologram 2, diffracting the laser beam of the laser diode 1 at a predetermined diffraction angle and transmitting the laser beam while reducing the divergence angle of the laser beam. A hologram disc 3 comprising at lest one hologram plate is rotatably mounted on a drive motor 4.

The hologram disc 3 diffracts the incident laser beam, outputted from the collimating hologram 2, at a predetermined angle, and in turn condenses and scans the diffracted laser beam to a reflection mirror 5. The reflection mirror 5 is disposed above the hologram disc 3 and reflects the laser beam scanned by the hologram disc 3 to a concave reflection mirror 6 where the incident laser beam is recondensed and reflected.

The laser scanning optical system further includes a wavelength compensating hologram 7 which compensates the movement of condensed position of the laser beam due to wavelength variation of the incident laser beam and condenses the compensated laser beam to the drum 8.

The laser scanning start point detecting optical system includes a laser scanning start point detecting hologram 9 which receives the laser beam, diffracted, condensed and scanned by the hologram disc 3 and reflected by the reflection mirror 5, at a position just before the laser beam is received by the concave reflection mirror 6. The detecting hologram 9 condenses the laser beam to a position spaced apart from its rear surface by a predetermined distance. A slit 10 is arranged at an intermediate position of the focus length of the detecting hologram 9 and passes the condensed spherical wave therethrough in the form of one point beam. The scanning start point detecting optical system further includes an optical detector 11 detecting the laser beam in the form of one point beam passing through the slit 10 and outputting a laser scanning start signal to the microprocessor 12.

Turning to Fig. 4, the hologram disc 3 includes a plurality of arcuate hologram pieces 3' attached on the annular surface of the disc one after another. As shown in Fig. 8, each of the hologram pieces 3' is manufactured by setting its laser beam condensing and diffraction angles under the condition that a diverged spherical wave W3, diverged from a position spaced apart from a holographic plate HP for the hologram pieces by a predetermined distance Flu3, and a condensed spherical wave WX, condensed to a position spaced apart from the holographic plate HP by a distance L, are applied to the holographic plate HP while being angled with a vertical axis of the plate HP at individual angles of i2 and eu3, respectively.

On the other hand, the collimating hologram 2 is manufactured, as shown in Fig. 7, by setting its collimating characteristics under the condition that a diverged spherical wave Wl, diverged from a position spaced apart from a holographic plate HP for the collimating hologram by a distance LH1, and another diverged spherical wave W2, diverged from a position spaced apart from the holographic plate HP by a distance LH2, are applied to the holographic plate HP while being angled with the vertical axis of the plate HP at an angle of e1, respectively The scanning start point detecting hologram 9 is manufactured, as shown in Fig. 9, by setting its scanning start point detecting characteristics under the condition that a diverged spherical wave W5, diverged from a position spaced apart from a holographic plate HP for the hologram by a distance LH5, and a condensed spherical wave W6, condensed to a position spaced apart from the holographic plate HP by a distance LH6, are applied to the holographic plate HP while being angled with the vertical axis of the plate HP at an angle of 8pj, respectively.

On the other hand, the wavelength compensating hologram 7 is manufactured, as shown in Fig. 10, by setting its wavelength compensating characteristics under the condition that a condensed spherical wave W7, received by a convex lens having the same focus length as that of the concave reflection mirror 6 at an angle of eH5 and in turn condensed to a position spaced apart from a holographic plate HP for the hologram 7 by a distance LHl, and a condensed spherical wave W8, transmitted through a cylindrical lens showing a power only in the YZ plane and having a focus length of LHg and condensed to a position, that is, the cylindrical lens focus, spaced apart from the holographic plate HP by a distance LHa, are applied to the holographic plate HP while being angled with the vertical axis of the holographic plate HP at the angle of respectively (YZ plane: the condensed spherical wave, XZ plane: a parallel wave) All of the compartments, including the above holograms, of the above optical systems are manufactured under the condition of following equations.

1) #H1 = e1, LH2 = L1 2) LH3 = LH1 + L2, LH4 > L3 + L4 + L5 + L6 #H2 = #2, #H3 = #3, 3) eH4 = elf LH6 = L8, LH5 = LH4, L7 < 4) #H5 = #4 = #5 = #6, LH7 = L5 + L6, LH8 = L6 wherein [I] L1 : a distance between the laser diode 1 and the collimating hologram 2, L2 : a distance between the collimating hologram 2 and the hologram disc 3, L2 : a distance between the hologram disc 3 and the reflection mirror 5, Lug : a distance between the reflection mirror 5 and the concave reflection mirror 6, L5 : a distance between the concave reflection mirror 6 and the wavelength compensating hologram 7, L5 : a distance between the wavelength compensating hologram 7 and the drum 8, and L7 : a distance between the reflection mirror 5 and the scanning start point detecting hologram 9.

[II] e : an incident angle of the laser beam to the collimating hologram 2, 82 : an incident angle of the laser beam to the hologram disc 3, 63 : a diffraction angle of the laser beam at the hologram disc 3, 84 : an incident angle of the laser beam to the concave reflection mirror 6, 8e : a reflection angle of the laser beam at the concave reflection mirror 6, and es : an incident angle of the laser beam to the wavelength compensating hologram 7.

In operation of the above holographic laser scanning unit, the laser beam generated by the laser diode 1 is received by the collimating hologram 2 at the angle of e.

Thus, the diverged spherical wave, diverged from the position spaced apart from the collimating hologram 2 by the predetermined distance LHl, is regenerated. The regenerated diverged spherical wave is received by the hologram disc 3 at the angle of 62. Upon reception of such an incident diverged spherical wave, the hologram piece 3' of the hologram disc 3 regenerates the condensed spherical wave, diffracted at the angle of 83 at a position spaced apart from the hologram piece 3' by the distance LH4. The regenerated condensed spherical wave is received by the concave reflection mirror 6 while being angled with the optical axis of the concave reflection mirror 6 at the angle of 94 and, thereafter, reflected by the mirror 6 at the angle of 65. The reflected laser beam is received by the wavelength compensating hologram 7 at the incident angle of 66. Upon reception of the condensed laser beam, the wavelength compensating hologram 7 regenerates the condensed spherical wave, condensed to the position spaced apart therefrom by the distance L8. The position to which the spherical wave is condensed is the drum surface, so that the laser beam condensed to the drum 8 moves along the X axis in accordance with rotation of the hologram disc 3. Hence, when the laser diode 1 is turned on by the microprocessor according to a video signal, a laser beam indicative of the video signal is scanned to the drum 8 to form a latent image according to the video signal on the drum surface.

When the laser diode 1 is turned on, the laser beam emitted from the diode 1 is varied in its wavelength due to, for example, a heat generated by the diode 1. Such a variation of the wavelength results in variation of both the incident angle 82 and the diffraction angle 83 of the laser beam of the hologram disc 3 and in turn causes movement of the condensed position of the laser beam on the drum 8 in the Z direction. In accordance with the present invention, such a movement of the condensed position of the laser beam is compensated by the wavelength compensating hologram 7.

Referring to Figs. 6 and 10, there is shown the wavelength compensating theory of the present invention.

Variation of the wavelength of the laser beam typically results in variation of the incident angle of the laser beam to the wavelength compensating hologram 7, so that the incident position of the laser beam to the wavelength compensating hologram 7 moves in the Z direction. However as shown in Fig. 10, the wavelength compensating hologram 7 of the present invention is manufactured by applying, employing the cylindrical lens showing the power only in the YZ plane, the condensed spherical wave Wa to the holographic plate HP.

In this regard, even though the laser beam, reflected by the concave reflection mirror 6 and applied to the wavelength compensating hologram 7, slightly moves on the compensating hologram 7 in the Z direction, the condensed position of the regenerated condensed laser beam is not varied. Thus, it is compensated the Z-directional movement of the laser beam on the drum 8 due to the wavelength variation of the laser beam and other vibration.

In addition, the laser scanning unit of the present invention should generate a signal indicative of video signal output timing. In order to achieve the above object, the laser beam, scanned by the rotating hologram disc 3 to the concave reflection mirror 6, is received by the scanning start point detecting hologram 9 prior to its incidence to the concave reflection mirror 6 as shown in Fig. 4. Upon reception of the scanned laser beam, the scanning start point detecting hologram 9 regenerates the condensed spherical wave condensed to the position spaced apart therefrom by the predetermined distance as shown in Fig. 9. Hence, the optical detector 11 is disposed at the position, spaced apart from the scanning start point detecting hologram 9 by the predetermined distance, with the slit 10 interposed between the scanning start point detecting hologram 9 and the optical detector 11.

Thanking for such an arrangement, the optical detector 11 receives the laser beam in the form of one point beam and detects the laser beam. Upon detecting the laser beam, the optical detector 11 generates a detecting signal which is in turn outputted to the microprocessor 12 as a signal indicative of video signal output timing.

In accordance with the aforementioned theory, the laser scanning unit of the present invention forms a high quality picture on the drum 8.

As described above, the present invention reduces the manufacturing cost of the laser scanning unit, a basic compartment of the laser printer, by substituting a hologram disc and a wavelength compensating hologram, both being mass producible, for the conventional rotating polygonal mirror and the compensating lens system which require difficult and complex manufacturing process resulting in involvement of capital investment. Furthermore, the use of the hologram disc improves the picture resolution.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. A holographic laser scanning unit for a laser printer comprising: a laser scanning optical system including: a laser diode for generating a laser beam under the control of a microprocessor in response to a printing video signal; a collimating hologram for diffracting said laser beam of the laser diode at a predetermined diffraction angle and transmitting sad laser beam while reducing a divergence angle of said laser beam; a rota'able hologram disc for diffracting the laser beam of said collimating hologram at a predetermined angle, and condensing and scanning the diffracted laser beam to a reflection mirror, said rotatable hologram disc including at least one hologram plate; a hologram dse drive motor; said reflection mirror for reflecting the laser beam scanned sy said hologram disc to a concave reflection mirror;; said concave reflection mirror for recondensing and reflecting the laser beam reflected by said reflection mirror; and a wavelength compensating hologram for compensating movement of a condensed position of the laser beam, reflected by said concave reflection mirror, due to wavelength variation of the laser beam and condensing the laser beam to a drum; and a laser scanning start point detecting optical system including: a laser scanning start point detecting hologram for receiving the laser beam, diffracted, condensed and scanned by said hologram disc and reflected by said reflection mirror, at a position just before said laser beam is received by said concave reflection mirror, and condensing the laser beam in the form of a condensed spherical wave to a position spaced apart from its rear surface by a predetermined distance; a slit for passing said condensed spherical wave, condensed by said laser scanning start point detecting hologram, therethrough in the form of one point beam, said slit being positioned at an intermediate position of a focus length of said laser scanning start point detecting hologram; and an optical detector for detecting said laser beam passing through said slit and outputting a laser scanning start signal to said microprocessor.

2. A holographic laser scanning unit according to claim 1, wherein said collimating hologram regenerates a diverged spherical wave having a divergence angle smaller than that of the laser beam generated by said laser diode.

3. A holographic laser scanning unit according to claim 1, wherein said wavelength compensating hologram is manufactured by employing a cylindrical condensing lens, showing a power only in a direction, in order to compensate movement of a condensed position of the laser beam due to a wavelength variation of said laser beam.

4. A holographic laser scanning unit according to claim 1 , wherein said hologram disc includes a plurality of arcuate hologram pieces attached on an outer annular surface of said hologram disc one after another.

5. A holographic laser scanning unit according to claim 1, wherein each of said hologram pieces is manufactured by applying a diverged spherical wave W3, diverged from a position spaced apart from a holographic plate by a predetermined distance LH3, and a condensed spherical wave W, condensed to a position spaced apart from said holographic plate by a distance LH4, to said holographic plate at individual angles of t2 and BH3 with respect to a vertical axis of said holographic plate, respectively;; said collimating hologram is manufactured by applying a diverged spherical wave W1, diverged from a position spaced apart from the holographic plate by a distance LHl, and another diverged spherical wave W21 diverged from a position spaced apart from the holographic plate by a distance LH2, to the holographic plate at an angle of eHl with respect to said vertical axis of the holographic plate;; said scanning start point detecting hologram is manufactured by applying a diverged spherical wave W5, diverged from a position spaced apart from the holographic plate by a distance LH5, and a condensed spherical wave Wg, condensed to a position spaced apart from the holographic plate by a distance LH6, to the holographic plate at an angle of e with respect to the vertical axis of the plate; and said wavelength compensating hologram is manufactured by applying a condensed spherical wave W7, received by a convex lens having the same focus length as that of said concave reflection mirror at an angle of EH5 and in turn condensed to a position spaced apart from the holographic plate by a distance LH7, and a condensed spherical wave Wg, transmitted through a cylindrical lens, showing a power only in a YZ plane and having a focus length of LH9, and condensed to a position spaced apart from the holographic plate by a distance LHa, to the holographic plate at the angle of i; with respect to the vertical axis of the holographic plate; and all of said holograms are manufactured under the condition of following equations #H1 = #1, LH2 = L1 LH3 = LH1 + L2, LH4 > L3 + L4 + L5 + L6 #H2 = #2, #H3 = #3, #H4 = #7, LH6 = L8, LH5 = LH4, L7 < L4 #H5 = #4 = e5 = e5, LH7 = L5 + L6, LH8 = L6 wherein L, is a distance between said laser diode and said collimating hologram, L2 is a distance between said collimating hologram and said hologram disc, L3 is a distance between said hologram disc and said reflection mirror, L4 is is a distance between said reflection mirror and said concave reflection mirror, L is a distance between said concave reflection mirror and said wavelength compensating hologram, Lj is a distance between said wavelength compensating hologram and said drum, L7 is a distance between said reflection mirror and said scanning start point detecting hologram, e1 is an incident angle of said laser beam of said collimating hologram, Q is an incident angle of the laser beam of said hologram disc, is is a diffraction angle of the laser beam, diffracted by said hologram disc, Q is an incident angle of the laser beam of said concave reflection mirror, e5 is a reflection angle of the laser beam, reflected by said concave reflection mirror, and #6 is an incident angle of the laser beam of said wavelength compensating hologram.

6. A holographic laser scanning unit for a laser printer substantially as hereinbefore described with reference to and as shown in Figs. 3 to 10 of the accompanying drawings.